Hi, I would guess that Peter is right that the 5mm dispersion could be increased at least to 10mm dispersion. We chose a 5mm dispersion chicane to match the upstream energy chicane; Woodley presented calculations on the emittance growth for the 5mm chicane in this talk from March 8,
And the emittance growth was negligible. Can do the appropriate scaling to check for 10mm dispersion chicane or even 20mm.
For a gas cherenkov counter I would agree with Peter that don't want to go narrower than 1cm-wide channels. If need to go narrower, can consider a quartz cherenkov counter though more susceptible to synchrotron radiation background. Should anyway consider a quartz counter for comparison.
One other item questioned at LCWS for the upstream polarimeter was how easy is it to get 35uJ/pulse in the ILC pulse format for the laser beam as designed; re. presentation regarding the Fabry-Perot cavity. Can you address this as well?
Also, this discussion would be very useful to have in the MDI forum on polarimetry at http://forum.linearcollider.org/.
I'll take some liberty and post this message with the thread below there. Cheers, mike

That makes indeed a lot of difference, to the detector, vacuum chambers,
etc. I had assumed 20 detector channels with a hor. aperture of 10 mm ea.,
covering a range of x = 20 -> 220 mm from the beam axis.

Scaling simply all x-dimensions by a factor 1/4 would give us very narrow
detector channels of only 2.5 mm nominal, which is not comfortable. Maybe
5 mm would be possible. Also, I don't want to get closer to the beam than
x_min = 20 mm. So that would decrease the maximum covered energy.

You indicate considerations about emittance growth as the origin of this
much more docile chicane design. This surprises me, since we have actually
investigated this point, based on earlier work by Nick Walker on the
TESLA energy spectrometer. If you look at page 18 of the talk Norbert
presented, we estimate an acceptabe emittance growth of 2.5%, but only
at E_cm = 1 TeV, and totally negligible at lower energies. So, unless
our estimate is bogus, it does not seem to be justified to decrease the
dispersion of the upstream chicane so drastically. My gut's feeling is
that we could absorb a factor of 0.5. But 0.25 would really hurt.

Even worse, and I would go as far as saying "unacceptable", would be
only 1.7 meters between the inner dipoles. We assumed a center to center
distance of 10 meters, which gives us nominally 8 meters of space for
a typical magnet length of 2 meters. The reason for this space requirement
is simple geometry for the laser beam insertion/exit. We went through this
exercise already with our Tesla design, and the arguments have not changed
and are still valid. You want to accomodate a crossing angle of 10 mrad,
and you must keep the optics away from any synchrotron radiation. These
conditions are met with our design, which accomodates a vertical beam
crossing, but retains a minimal magnet gap height of only 20 mm for all
dipoles. With a space of only 1.7 m, you would either have to increase
the height of the gap by some unacceptable factor, or you would have to
resort to horizontal beam crossing, which will jeopardize the optics and
is therefore also not acceptable.

I have not had time to look at all the details of the current beam line
elements, but I will take a closer look. Obviously, there are already
enough issues of contention. Clearly the actual amount of emittance growth
in the chicane should be checked with high priority.